Why did he do it? Kevin Townsend explains at The Atlantic:
I had to know what had compelled Johnson to start rocking those locks. And, as it so happens, a search revealed the explanation in a story from the July 1973 issue of this very magazine:
So Johnson suffered the election in silence, swallowing his nitroglycerin tablets to thwart continual chest pains, endorsing McGovern through a hill country weekly newspaper, meeting cordially with the candidate at the ranch. The newspapers showed a startling picture of Johnson, his hair almost shoulder-length. Former aide Bob Hardesty takes credit for this development. “We were working together one day,” Hardesty recalls, “and he said, in passing, ‘Robert, you need a haircut.’ I told him, ‘Mr. President, I’m letting my hair grow so no one will be able to mistake me for those SOB’s in the White House.’ He looked startled, so I explained, ‘You know, that bunch around Nixon—Haldeman, Ehrlichman—they all have very short hair.’ He nodded. The next time I saw him his hair was growing over his collar.”
Read the entire piece.